Electric double-layer capacitors that can be electrically charged and discharged with large currrents are promising as electric power storage devices that are charged and discharged frequently such as electric vehicles, solar battery-assisted power supplies, and wind power-assisted power supplies. Therefore, there is a demand for electric double-layer capacitors having high energy densities, being capable of being quickly charged and discharged, and providing excellent durability (for example, see 4th EV/HEV Symposium on State of the Art—Present Circumstances of Capacitor Technology and Forthcoming Problems “International Symposium on State of the Art of Batteries for Electric Vehicles”, Executive Committee, Nov. 8, 1999.)
In such an electric double-layer capacitor, a pair of polarizing electrodes are placed opposite to each other via a separator within an electrolyte solution, thus forming positive and negative electrodes. The principle is that electric charge is accumulated in an electric double layer formed at the interface between each polarizing electrode and electrolyte solution. It has been considered that the capacitance of the electric double-layer capacitor is roughly proportional to the area of the polarizing electrodes. Therefore, only activated carbon having large specific surface areas (i.e., having micropore diameters of more than about 2 nm) has been used as the active material for polarizing electrodes in the past (e.g., Patent Laid-Open No. 2002-15958).
In contrast, the present inventors and others have proposed electric double-layer capacitors having excellent capacitances and withstand voltages and using the conventionally employed electrolyte and carbon materials having characteristics entirely different from those of the aforementioned activated carbon (Patent Laid-Open Nos. H11-317333, 2000-77273, and 2002-25867).
Research is also underway on electrolytes. Wilkes and others have announced that ethyl methylimidazolium (EMI) salts have excellent thermal stability and high ionic conductivity as liquid electrolytes which are liquids at room temperature (also known as room temperature molten salts or ionic liquids), and that the EMI salts are liquids which are stable even in air (John S. Wilkes et al., J. Chem. Soc., Chem. Commun., 1992, pp. 965–7). Furthermore, Carlin and others have announced that AlCl4− salts and BF4− salts of 1-ethyl-3-methylimidazolium (EMI) and 1,2-dimethyl-3-propyl-imidazolium (DMPI) act as electrolytes and, furthermore, intercalate/deintercalate from graphite electrodes electrochemically and thus act as a simple battery (DIME battery) (Richard T. Carlin et al., J. Electrochem. Soc., Vol. 141, No. 7, pp. L73–L76 (1994)).
Then, various attempts have been made. In known techniques, the above-described 1-ethyl-3-methylimidazolium (EMI) is used as the liquid electrolyte in electric double-layer capacitors using activated carbon or EMI is dissolved in an aprotic organic solvent and used as an electrolyte (Patent Laid-Open No. 2002-110472 and U.S. Pat. No. 2,945,890). Another known liquid electrolyte assumes a quaternary ammonium salt structure (Patent Laid-Open No. H11-297355). In further known liquid electrolytes, the substituent group of imidazolium has been replaced (Patent Laid-Open Nos. 2002-175948 and 2002-222740).